Such a resonant circuit is disclosed for instance, in an article by W. A. Tabisz, P. Gradzki and F. C. Lee published in the IEEE Power Electronics Specialists Conference Record, 1987, p. 404-413, Zero-Voltage-Switched quasi-Resonant buck and flyback Converters-Experimental results at 10 MHz. In general, resonant LC circuits using switching means as well as diodes have been employed for a variety of purposes at various frequencies and at relatively high or low power levels. In electronic applications based on the use of integrated circuits, one field having substantially relied on resonant and so-called quasi-resonant circuits is that of power supply circuits, particularly those using power MOSFETs at high frequencies, e.g. 10 MHz. The above article is an example of such DC/DC converter developments.
While many resonant-mode topologies have been defined for such High Frequency DC/DC converters using MOSFETs, the described buck ZVS-QRC uses the initially defined resonant circuit as a quasi-resonant MOSFET gate drive. This even more particular application of such resonant circuits is especially significant as turning a MOSFET on/off requires charging/discharging the input capacitance appearing between its gate and source bringing significant power dissipation, proportional to the frequency, and switching speed problems, Using another lower power MOSFET, driven by a non-resonant gate drive, as switching means between the gate and the converter reference potential connected to the source through the converter flywheel diode, when this control MOSFET is off the controlled converter power MOSFET is on. Upon the control MOSFET being turned on, the gate potential of the controlled power MOSFET practically drops to the common reference potential whereas its source potential remains momentarily higher due to the parasitic capacitance of the flywheel diode. Hence, this negative bias between gate and source reduces the turn-off time of the controlled power MOSFET. While the control MOSFET is on, the inductance current increases linearly but this peaks off upon the control MOSFET being turned off, thereby setting up the series resonant circuit formed by the inductance and the controlled power MOSFET input capacitance. Upon the inductance current returning to zero, resonance is stopped by a diode in series with the inductance becoming blocked, the gate to source potential reaching the necessary potential for the controlled power MOSFET to be turned on in a resonant manner reducing the power dissipation by half as compared to conventional gate drives. As for the above control MOSFET, these drive circuits can rely on bipolar transistors, gates and other integrated circuits.
With other topologies or applications in which the source of the power MOSFET exceeds 20 volts with respect to the gate drive ground, or in which the source is directly connected to the reference ground, a negative gate drive voltage cannot be obtained. In addition, DC isolated. MOSFET gate drive circuits are needed if the MOSFET source is momentarily above 20 volts with respect to the gate drive ground, e.g. the article by J. D. Repp in the May 1989 Proceedings of the High Frequency Power Conversion Conference, p. 438-445, Ultra fast isolated gate drive circuit. It is pointed out therein that transformer isolated power MOSFET gate drives should possess various desirable properties including low power consumption. For the described 100 watt converter, a consumption of 2.5 watt is deemed low but since this is at 500 kHz only, it is clear that the losses can be quite significant at substantially higher frequencies and indeed, the author states there is room for further improvement in this respect.
Thus, the quasi-resonant gate drive of the first article uses a common reference or ground potential for the power MOSFET input capacitance and for the converter whereas the second article indicates this is not suitable for other converter topologies requiring galvanic isolation provided by a transformer. Moreover, the gate drive of the first article only provides a positive potential across this input capacitance and a negative potential from gate to source is only obtained by a positive potential remaining temporarily at the source with respect to ground, due to the junction capacitance of the flywheel diode used in this buck converter. This resulting negative gate bias is on the other hand desirable for MOSFET turn-off. In addition, this circuit only saves one half of the total gate power as compared to the conventional gate drive. Further, this known circuit implies that the capacitance is charged at the desired positive gate level to turn the power MOSFET on up,on a series diode blocking the resonant current as it decreases to zero, the energy stored in the inductance having then been transferred to the capacitance. Any variation in this energy, i.e. tolerances, will lead to insufficient or excessive voltage across the capacitance.